Copy a featured image from current site to blog in the network

set_post_thumbnail_multisite.php

<?php

// Let us make sure we are on the curent site.
restore_current_blog();

// get the image src
$image_url = wp_get_attachment_image_src( $_POST['featured_image'], 'full' );
$image_url = $image_url[0];

// Now let us switch to the Blog we like to set the featured image
switch_to_blog( $buddyforms[ $form_slug ]['blog_id'] );

// Add Featured Image to Post
$upload_dir = wp_upload_dir(); // Set upload folder
$image_data = file_get_contents( $image_url ); // Get image data
$filename   = basename( $image_url ); // Create image file name

// Check folder permission and define file location
if ( wp_mkdir_p( $upload_dir['path'] ) ) {
  $file = $upload_dir['path'] . '/' . $filename;
} else {
  $file = $upload_dir['basedir'] . '/' . $filename;
}

// Create the image  file on the server
file_put_contents( $file, $image_data );

// Check image file type
$wp_filetype = wp_check_filetype( $filename, null );

// Set attachment data
$attachment = array(
  'post_mime_type' => $wp_filetype['type'],
  'post_title'     => sanitize_file_name( $filename ),
  'post_content'   => '',
  'post_status'    => 'inherit'
);

// Create the attachment
$attach_id = wp_insert_attachment( $attachment, $file, $post_id );

// Include image.php
require_once( ABSPATH . 'wp-admin/includes/image.php' );

// Define attachment metadata
$attach_data = wp_generate_attachment_metadata( $attach_id, $file );

// Assign metadata to attachment
wp_update_attachment_metadata( $attach_id, $attach_data );

// And finally assign featured image to post
set_post_thumbnail( $post_id, $attach_id );